In recent years, a wavelength division multiplexing (WDM) communication has been known as a communication method that achieves a large volume data communication. In the wavelength division multiplexing communication, a plurality of optical signals having wavelengths different from one another are multiplexed and then transmitted. The wavelength division multiplexing communication is used also for a long distance optical transmission system such as an optical submarine cable system.
In order to obtain uniform transmission characteristics for the entire transmission band in the long distance optical transmission system such as the optical submarine cable system, management of the wavelength dependency of an insertion loss that an apparatus arranged in a transmission path and components of this apparatus have is important. A reconfigurable optical add/drop multiplexer (ROADM) function capable of dynamically changing the path route of the network has recently been applied also to the optical submarine cable system. A wavelength selective switch is known as an optical device that achieves the ROADM function.
Regarding the wavelength selective switch, Patent Literature 1 discloses an optical transmission apparatus in which a wavelength selective switch is used along with an optical channel monitor (OCM). In Patent Literature 1, the optical channel monitor includes a wavelength dividing unit and an intensity detector. The wavelength dividing unit includes a wavelength tunable filter that allows an optical signal of a specific wavelength to pass therethrough, and divides the optical signal for each wavelength. The intensity detector includes a light-receiving device. The intensity detector receives optical signals that have been divided by the wavelength dividing unit and detects the light intensity of the optical signal for each wavelength. The optical transmission apparatus disclosed in Patent Literature 1 controls the amount of attenuation for each wavelength in the wavelength selective switch in such a way that the intensity detected for each wavelength using the optical channel monitor reaches a target value.